1. Field of the Invention
The present invention relates to an interference apparatus for detecting the state of a wave surface capable of speedily and accurately detecting phase differences as between beams of reference light and test-sample light over a certain range and relates more particularly to an interference apparatus for detecting the state of a wave surface capable of detecting with consistently high accuracy the phase difference as between beams of reference light and test-sample light even when the output from a light source fluctuates.
2. Description of the Prior art
Various interferometers such as Twyman interferometers and Mach-Zehnder interferometers have been utilized in measuring optical performance and specifically wave front aberrations of optical devices such as image pickup lenses. Apparatuses of these kinds include a so-called phase modulation interference type of measuring apparatus which measures the optical performance of an object of measurement by detecting the phase difference between beams of reference light and test-sample light which have passed through the object, that is, detecting interference fringes in a time division manner. This type of apparatus has generally been used because they normally have a simple form of construction and are capable of performing measurement with high accuracy. They detect in time division manner brightness variations of interference fringes which are formed on a light receiving surface when, for example, a reflecting mirror and the like disposed on the optical path for reference light is vibrated to a minute degree. Accordingly, brightness variations which are caused by factors other than the change of the optical path act as noises and reduce the measurement accuracy. Specifically, in this type of apparatus, the output of a light source must be maintained at a constant level. It has thus been difficult for a light source having varying output to be adapted for use in this type of apparatus.
However, a light source having a specific wave length corresponding to that of an object device has sometimes been needed if the performance of the object of measurement is to be measured at its specific operating wave length even when the output of the light source is to some extent unstable.
For the purpose of measuring the performance of the object of measurement in, for example, an ultraviolet radiation range, it is effective to use an eximer laser because it is capable of emitting various spectra of light in the ultraviolet radiation range and of readily outputting this light at a high level. The output of the eximer laser, however, is unstable, so that, for the above-described reason, this laser reduces the accuracy of a measuring apparatus of the phase modulation interference method when directly incorporated in this apparatus.